The present invention relates generally to television receivers, and particularly to improvements in synchronous detection systems for use in television receivers.
Present high quality television receivers generally employ a synchronous detector rather than a diode detector for detecting video signals. The advantages of synchronous detection are well known and need not be detailed. Suffice it to say that the resultant picture which the receiver generates is of a higher quality than that obtainable by use of a diode detector.
As is well known, a synchronous detector receives a 45 megahertz video signal and an oscillator signal which is of exactly the same frequency and of substantially the same phase as the video signal. To develop the oscillator signal, a phase-locked loop is usually employed wherein a limiter limits the amplitude of the video signal and applies the limited video signal to a phase detector. The phase detector also receives an oscillation signal from a VCO (voltage controlled oscillator) and generates an error signal for locking the VCO to the frequency of the video signal. When such frequency lock occurs, the phase of the oscillator signal is also generally locked such that it is in phase quadrature with the video signal received by the phase detector. As a result of such quadrature phase lock, the oscillator signal must be shifted in phase by ninety degrees to bring it into alignment with the phase of the video signal which is received by the synchronous detector, thereby to provide the proper detection axis for the synchronous detector.
One way in which the detection axis of the synchronous detector has been at least partially adjusted is by modifying the phase response of the limiter. To accommodate such modification, the limiter conventionally includes a variable tuned circuit which serves multiple purposes. One of those purposes is to modify the phase of the limited video signal so that the VCO locks at a phase at which an acceptable detection axis occurs. Another purpose of the tuned circuit is to adjust the limiter for optimum differential phase performance. However, adjusting the tuned circuit for optimum performance of one of its functions causes another of its functions to be performed less than optimally. For example, when the tuned circuit is adjusted for the best detection axis, its differential phase performance may suffer. Likewise, when the tuned circuit is adjusted for best differential phase performance, the VCO may be locked to a phase which produces an undesirable detection axis. Hence, the adjustment of the tuned circuit has been a compromise among conflicting requirements, none of which end up being met optimally. Hence, the qualities inherent in synchronous detection have not been taken advantage of to the fullest desirable extent.